1. Field of the Invention
The present invention relates to an optical recording medium that includes a recording layer composed mainly of an organic compound, and to an optical recording/reproducing method using the optical recording medium.
2. Disclosure of the Related Art
Rewritable media for optical recording of information, such as CD-Rs (compact disk-recordable) and DVD-Rs (digital versatile disk-recordable), have become widely used. Such recording media include a recording layer that makes use of an organic dye. CD-R, a recording medium that permits the use of near infrared laser light in recording/reproduction of information, offers advantageous characteristics, such as low prices and ability to keep recorded information from being altered, that have made this recording medium widely accepted in the marketplace. In response to an increasing demand for high-density recording medium that offers long recording time, DVD-Rs have been developed and become increasingly popular. DVD-Rs enable long-time recording by permitting the use of infrared laser light in recording/reproducing of information: DVD-Rs can utilize recording/reproducing laser light with a wavelength of 650 nm, as compared to 780 nm used in CDs, and optical systems employed in DVD-Rs achieve a numerical aperture (referred to as ‘NA,’ hereinafter) of 0.6, as compared to 0.45 for CDs. In this manner, DVD-Rs have achieved a large recording capacity (4.7 GB/per side), which is 6 to 8 times larger than that of CDs and allows about 2-hour recording time when typical TV signals are recorded.
To meet a demand for recording media with even higher recording density, efforts have been made to further decrease the wavelength of the recording/reproducing laser light and increase the numerical aperture of the optical system employed. For example, an advanced optical recording medium is proposed for use in a system in which blue-violet semiconductor laser light (390 to 420 nm in wavelength) is used as the recording/reproducing laser light in conjunction with a lens system with an NA of 0.76 or higher. The use of the short wavelength laser light is expected to bring about a significant increase in the recording density of optical recording media.
Various organic dye materials have been proposed for use in the recording layers of CD-Rs and DVD-Rs. Some of these materials that have already been put to practical use are such that the long wavelength region of their absorption spectrum corresponds to the wavelength of the recording/reproducing laser light and the requirements for both the high refractive index (n>2.0) and the proper extinction coefficient (0.01<k<0.10) are met in the long wavelength region. When the recording laser light is irradiated onto the recording layer, the organic dye material absorbs the light and as a result, melts or degrades, which in turn causes the refractive index of the recording layer to decrease from a relatively high value to a relatively low value. As a result, recording pits are formed to record information. To subsequently reproduce the recorded information, the reproducing laser is shone onto the recording layer and the information is read by taking advantage of the difference in the reflective index with respect to the laser light between the recording pit and the surrounding non-recording area.
CD-Rs and DVD-Rs are both required to have a high reflective index in order to ensure compatibility with CD-ROMs (read-only memory) and DVD-ROMs, both of which have a high reflective index. However, the organic dye material that has a refractive index of 2<n<3 and an extinction coefficient of 0.01<k<0.10 cannot provide a desired high reflective index by itself. For this reason, CD-Rs and DVD-Rs include a highly reflective metal reflection layer on one side of the recording layer opposite from the side exposed to the laser light. In short, for the purpose of ensuring a high reflective index and a high modulation and thus ensuring the compatibility with ROMs, CD-Rs and DVD-Rs both employ an organic dye material that, when illuminated with light in the wavelength range of the recording/reproducing laser, melts or degrades to change its refractive index from a relatively high value to a relatively low value.
The advanced optical recording media, in which blue-violet semiconductor laser light (390 to 420 nm in wavelength) is use as the recording/reproducing laser light, generally have a low reflective index, since there is difficulty in principle in imparting a high reflectance comparable to that of ROMs to the rewritable media (RW), which use a phase-change material in their recording layers. Thus, if a rewritable optical recording medium is developed that, unlike CD-Rs and DVD-Rs, does not require a high reflective index, it will become possible to use, in the recording layer of the recording medium, an organic dye material that, when irradiated with laser light, melts or degrades to cause the refractive index to change from a relatively low value to a relatively high value. This possibility is suggested in Japanese Patent Laid-Open Publication No. 2001-273672.